GRAHAM MARTIN, the last U.S. ambassador in Sai-

gon, and the man upon whom The Last Ambassador is based, was a memorable figure. You felt on meeting him that here was the consummate diplomat, soft- spoken and almost courtly on the surface, but underneath cold and calculating to an extraordinary degree.

It was the habit among correspondents in Saigon at the time to look down one's nose at Martin, as if he were slightly defective in some way. He seemed hawkish on the war, and was strongly anticommunist. More than that, I think, one had the sense that he was manipulating things behind the scenes and not telling us, or worse, lying to us. One felt left out, and even more irritating, Martin made no bones about the fact that he regarded many newsmen as moral weaklings or Comsymps, or both. For all I know, he may have been right.

Despite the force of Martin's personality, the American people decided to cut their losses in Indochina. Congress curbed the flow of war funds, the Army of the Republic of Vietnam (ARVN) collapsed before a broad offensive, and on April 30, 1975 the North Vietnamese captured Saigon. Frank Snepp, the CIA man who later wrote a book critical of Martin's handling of the last days, told me that as his rescue helicopter lifted out of the U.S. embassy compound after dark, he looked up the Bien Hoa highway to the north and saw a solid stream of headlights for 20 miles--the highly mechanized North Vietnamese Army rolling into town.

I had taken a chopper that left at dusk. You could see a ring of fires and explosions around the dying city. It still haunts me, but the tears and screams and tormented faces have faded now into the past. I can't even remember if I actually saw him--Martin--out there on the South China Sea where the American fleet gathered to pick up the remnants of our adventure in Indochina. Maybe it was a faded gray newsclip I saw--that shattered, bedraggled figure of America's last ambassador in Saigon.

Now NBC News diplomatic correspondents Bernard Kalb and Marvin Kalb have taken the strong figure of Martin and this rich backdrop of the fall of Saigon to shape their novel, which a publicity blurb describes as "a tragic story of love, war, and betrayal." The last ambassador in this book is named Hadden Walker, but in many other respects resembles Martin. Walker is a southern gentleman, a tough, manipulative career diplomat determined to keep South Vietnam afloat by whatever means available in order to salvage "at least a scrap of American honor," as he puts it. Snepp also appears to be here, in the form of Tony Catlett, a hard-nosed young CIA man. Catlett is the lover of the fictional ambassador's daughter, Suzanne, who is living with her father in Saigon. Catlett is convinced that the only way to prevent the impending disaster is to negotiate a coalition government with the North--anathema to Ambassador Walker. In a flight of fancy, the Kalbs invent a smashing Eurasian mistress for their fictional last ambassador, thus endowing him with a human weakness that, so far as I know, nobody ever accused the real last ambassador of having.

The plot turns on the ambassador's efforts to prevent the inevitable fall of Saigon. Catlett pressures him to try to negotiatie a coalition, arguing, "It's over, except for the finale. The only question now is what kind of a finale. A coalition government in which the Communists share power? Or an all-out NVA victory, American defeat?" But the ambassador doesn't see it that way. To him, the North Vietnamese offensive can be stopped if Washington would only live up to its word and provide enough military aid. "With emergency aid, Saigon has a chance," the ambassador explains to his daughter in what is--for this didactic blend of fact and fiction--an intimate conversation. "Not a guaranteed chance; no man in his right mind would guarantee anything about this country. But a chance. That's why I am . . . polishing ARVN's image, reassuring Washington that ARVN is in there scrambling, so Washington can't use the excuse of a 'hopeless situation' as a way out of its reponsibility." While the ambassador defends his misrepresentations as "role playing" in a "real world that doesn't offer you the luxury of truly ethical choices," his daughter immediately perceives that his effort to withhold the full story from Washington amounts to lying and that her father has been corrupted by the war.

The Kalbs' psychological analysis of the motives of the last ambassador to Saigon is, if read as a portrait of Graham Martin, the most sympathetic one of him that I have seen. After all, Martin's was an impossible situation. The Kalbs also show how Henry Kissinger and Gerald Ford covered their flanks by using Martin as a scapegoat.

As fiction, you couldn't call The Last Ambassador rich, good reading. The writing is bland, strangely devoid of feeling for such an explosive subject, perhaps because the Kalbs weren't there in Saigon at the end. If you started gathering up all the thighs and nipples in the repetitive, mechanical sex scenes, you would soon get a basketfull. But as a footnote to diplomatic history, the book is an interesting, fanciful contribution. At least it reminds us that the Vietnamese were people we led down the garden path and then betrayed terribly. In that outing, we did not cover ourselves with glory.